package com.exam.jingdong;

import java.util.Scanner;

/**
 * date:
 * description:
 */
public class Solution001 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int numTanks = scanner.nextInt();
        int health = scanner.nextInt();
        int attack = scanner.nextInt();
        int numTowers = scanner.nextInt();
        int totalHealth = health * numTowers;
        int res = 0;
        while(true){
            res++;
            totalHealth -= numTanks;
            if(totalHealth <= 0){
                System.out.println(res);
                break;
            }
            numTowers = totalHealth / health;
            if(totalHealth % health != 0){
                numTowers++;
            }
            numTanks -= numTowers * attack;
            if(numTanks <= 0){
                System.out.println(-1);
                break;
            }
        }

    }
}
